It is well known that circuit packaging can strongly influence the properties of integrated circuits (ICs). Lowering the dielectric constant (K) of the substrate materials in ICs increases signal propagation velocity, reduces power consumption and minimizes electronic cross-talk, thus allowing for higher functional density. Polymeric materials typically offer lower dielectric constants than ceramic materials, but for many applications the reliability, thermal stability, and superior thermal conductivity of ceramic packages make them more desirable. The low-K ceramic dielectrics currently in use are predominantly filled glass systems in which the glass is typically a borosilicate or aluminosilicate composition and the fillers are typically crystalline silicates. One important function of these fillers is to improve the thermal coefficient of expansion (TCE) matching of the dielectric to the other components of the circuits. The TCE of the dielectric is normally matched either to that of alumina (.apprxeq.6.2 ppm/.degree.C.) or silicon (.apprxeq.3.5 ppm/.degree.C.).
Often, the glasses used in circuit packaging have TCEs less than those required to match the TCE of the substrate or of the other components in the circuit. In these instances, the filler materials should have TCEs greater than about 4-6 ppm/.degree.C., preferably in the range of about 6-12 ppm/.degree.C., so that the composite TCE of the dielectric can be matched to the substrate and/or other components. Fillers used in composite packaging systems should also have low dielectric constant, good chemical compatibility and should sinter to dense bodies in combination with the glasses of interest.
For low dielectric constant applications, quartz is the most commonly used high-TCE filler. Quartz has a desirably low dielectric constant (about 4-4.5), a TCE of about 10-12 ppm/.degree.C. in the temperature range 25.degree.-300.degree. C., and excellent chemical durability. However, quartz undergoes a displacive alpha-to-beta phase transition at 573.degree. C., which is accompanied by a relatively large volume change. It is wellknown that this abrupt volume change can cause mechanical instabilities (cracking, crazing, etc.) to develop in multilayer systems upon thermal cycling through the phase transition. Although the use temperature of the package does not normally exceed 150.degree.-250.degree.C., a large number of re-firings at 850.degree.-950.degree. C. are typically necessary to fabricate a ceramic multilayer electronic package, so the alpha-to-beta phase transition is a disadvantage is quartz-filled systems. Quartz is also not wet well by many glasses used in low-K dielectrics, so the sintering properties of quartz often limit its loading in filled glass composites. When the loading becomes too high, the filled glass dielectric composites do not sinter hermetically, and the dielectric is no longer a good insulator because of its open porosity. A filler with improved glass wetting properties would allow higher loadings of low-K filler, reducing the dielectric constant of the composite and increasing its hermeticity.
There are two crystalline forms of silica other than quartz which are stable at ambient pressure, tridymite and cristobalite. Each of these structural forms undergoes a displacive alpha-to-beta phase transition accompanied by an abrupt volume change, and thus suffers from the same mechanical instabilities as quart does. If the high-temperature forms of quartz, tridymite, or cristobalite could be stabilized to room temperature, the stabilized material would not suffer from phase transitions upon heating or cooling. It has been recognized for a number of years that glass-ceramics in which the predominant crystalline phase is stabilized high (beta) cristobalite can be formed by crystallizing a high-silica glass containing other constituents such as Na.sub.2 O, CaO, and Al.sub.2 O.sub.3. This approach to forming stabilized high cristobalite glass-ceramics was disclosed by MacDowell (U.S. Pat. No. 3,445,252), who claimed that glass compositions containing 55-90 weight % SiO.sub.2, 5-40 weight % Al.sub.2 O.sub.3, (SiO.sub.2 /Al.sub.2 O.sub.3 =5.0-7.1) and 1-5 weight % CaO, CuO or SrO crystallized to glass-ceramics with high cristobalite as the primary crystalline phase. Li (U.S. Pat. No. 4,073,655) claimed high cristobalite glass-ceramic articles with improved phase purity by restricting the glass compositions to those containing equimolar ratios of CaO (up to 70 mole % of the CaO can be substituted by other oxides) and Al.sub.2 O.sub.3.
Kaduk (U.S. Pat. No. 4,395,388) disclosed the formation of stabilized high cristobalite from a high pH solution containing silica and a source of boron oxide which was reacted under hydrothermal conditions in the presence of glycerol. This type of process normally forms low (alpha) cristobalite. Recently, Perrota et al. (U.S. Pat. No. 4,818,729) claimed another wet chemical process for forming stabilized high cristobalite from dried gels containing silica, Al.sub.2 O.sub.3, and any alkali or alkaline earth oxide, excluding Li.sub.2 O, BeO, and MgO. They claim, as previously described by Li, that the molar ratio of Al.sub.2 O.sub.3 to alkali or alkaline earth oxide must be nearly equimolar, i.e. between 0.95 and 1.1, or the high cristobalite phase is not stabilized. They specify that the ratio of silica to Al.sub.2 O.sub.3 can vary from 10-40 to form the stabilized material. Their process requires calcination at 800.degree.-1400.degree. C. for long times (often greater than 24 hours) to form high cristobalite and yielded material which was contaminated by other phases, such as anorthite. It would be greatly preferred to shorten the required crystallization time so that the synthesis process would be more economical.
The mechanism of the stabilization of high cristobalite is a matter of some controversy. Various workers have proposed that stabilization is achieved by "stuffing" interstices in the cristobalite framework with mono- or divalent cations. These cations are charge-compensated by the substitution of Al.sup.3 + for Si.sup.4 + in the framework. Other authors have stated that stabilized cristobalite is formed when crystallization of cristobalite occurs in a glass matrix which constrains the crystalline particles and prevents the inversion to the low-temperature (alpha) form of cristobalite. Finally, other authors have found that stabilization may be achieved by the introduction of defects or stacking faults into the cristobalite structure. These defects may take the forms of tridymite-like intergrowths. It is unclear at this time which, if any, of these mechanisms are operative.
To be useful as a filler in composite dielectrics, a material should have smooth thermal expansion behavior, i.e. it should not undergo any abrupt volume changes up to at least 1000.degree. C., and should have a TCE of about 6-12 ppm/.degree.C. in the range 25.degree.-300.degree. C. The filler must also be stable in the presence of molten glasses, a condition which occurs during firing. Of course, the dielectric constant of the material should be as low as possible, preferably less than about 5, and the dielectric loss should be less than about 0.5% tan d. Finally, the filler powders should have good wetting properties in combination with a variety of glasses, in order to form dense composites at filler loadings of at least 20-60 volume percent. In particular, the wetting properties of the filler should allow larger filler loadings than are possible with quartz. Phase-pure materials are preferable for packaging applications, but materials containing small amounts of impurity phases may also be suitable, so long as the predominant phase has the desired properties and the properties of the impurities are not deleterious.